1. Field of the Invention
The present invention relates to a monitor region extension member which is an optical member for acquiring, when image input means are used to acquire optical images of a monitor region surrounding a motor vehicle, optical images of an extended monitor region which is a predetermined region surrounding the monitor region, in addition to the optical images of the monitor region, and relates to a motor vehicle peripheral monitoring apparatus using the monitor region extension member to monitor a peripheral region of a motor vehicle which is difficult for the driver to view.
2. Description of the Related Art
FIG. 12A is a perspective view for explaining the monitor region extension member of the prior art, and FIG. 12B is a diagram for explaining the shaded portion which may occur in a picture formed by the CCD camera.
Known as the prior art for extending the monitor region for the back monitor are ones of types shown in FIGS. 12A, 12B, 13A, and 13B.
As shown in FIG. 12A, such a type of conventional monitor region extension member 2 and the motor vehicle peripheral monitoring apparatus using the same were constructed as an apparatus for monitoring the periphery, especially lateral sides and backward, which are difficult for the driver to view, of a motor vehicle such as an automobile, a bus or a truck.
In order to provide an apparatus for imparting information on the peripheral region to the driver, for instance, on the rear part of the motor vehicle there was mounted an image forming device such as a CCD camera for imaging the backward of the motor vehicle. The peripheral region was imaged by the image forming device and the thus formed images were displayed on a monitor or the like.
To secure an enhanced safety, it would be useful for such a motor vehicle peripheral monitoring apparatus to extend the imaged region to enlarge the region to be monitored.
For this reason, in the conventional motor vehicle peripheral monitoring apparatus, a monitor region extension member 2 for extending the monitor region was fixedly disposed immediately in front of a lens 4 of the CCD camera.
The monitor region extension member 2 comprised as shown in FIG. 12A a prism portion 2A in the form of a prism with a curvature through which images of a first predetermined region containing an extended monitor region optically pass into the lens 4 of the CCD camera, a curved portion 2B for optically compressing images of a second predetermined region continuous with the monitor region side of the first predetermined region and inputting the thus compressed images into the lens 4 of the CCD camera, and a planar portion 2C for facilitating handling of the monitor region extension member 2 by removing the sharp edges from the monitor region extension member 2.
Furthermore, in the monitor region extension member 2, the prism portion 2A was formed from a prism having a constant prism apical angle. The curved portion 2B was comprised of a cylindrical surface whose prism apical angle is set to be smaller continuously from the prism apical angle of the prism portion 2A.
By utilizing the refracting effect of the prism, the motor vehicle peripheral monitoring apparatus using the thus constructed monitor region extension member 2 was able to extend the monitor region for a single CCD camera upward from the center, with the aid of a prism with a curvature (that is, the prism portion 2A) as shown in FIG. 12A inserted into the optical system of the CCD camera, where the monitor region is a region extending far from the rear vicinity of the motor vehicle and the extended monitor region is a rear far regionoutside the monitor region. Furthermore, the curved portion 2B was able to optically compress images of a region continuous with the rear far region but excluding the rear near region, and to input them into the lens 4 of the CCD camera.
FIG. 13A is a perspective view for explaining the optical positional relationship between the monitor region extension member of FIGS. 12A and 12B and the CCD camera, and FIG. 13B is a sectional view taken along a line Q.sub.A -P.sub.A in FIG. 13A.
In such a monitor region extension member 2 and the motor vehicle peripheral monitoring apparatus using the same, as shown in FIG. 13A, it was possible to increase the field angle .theta..sub.MAX in the diagonal direction (that is, the direction of axis Q.sub.A -P.sub.A) relative to the field angle .theta..sub.MIN in the vertical direction (that is, the direction of Y-axis) at the central portion.
That is, it was possible to extend the monitor region since there could be increased the angle of incidence .beta..sub.A of ray output from the prism portion 2A toward the lens 4 as shown in FIG. 13B.
In general, due to the existence of optical distortion in the lens 4 of the CCD camera, the actual image forming position appears closer to the center of the lens 4 accordingly as the image forming position comes away from the central axis (that is, optical axis) of the lens 4.
More specifically, in the imaging test using a lattice plate having square lattices (a lattice pattern), the field angle became larger accordingly as it came away from the center of the lens 4, with the result that the images (in this case, imaging of the lattice pattern) obtained by the CCD camera was optically converted into pictures which were bulged in both vertical and horizontal directions (that is, distorted pictures) compared with the actual lattice pattern.
However, a monitor region extension member 2 and the motor vehicle peripheral monitoring apparatus using the same entailed a technical deficiency in that it was possible for the field angle .theta..sub.MIN to extend the monitor region without causing any total reflection, but that for the field angle .theta..sub.MAX the input angle .beta..sub.A tends to exceed the critical angle I.sub.c (=arcsin (n.sub.0 /n.sub.1), see FIG. 13B), resulting in frequent occurrence of the total reflection.
It would also be conceivable to decrease the prism apical angle of the monitor region extension member 2 in order to eliminate the total reflection (that is, .beta..sub.A .ltoreq.I.sub.c) caused by the increase of the field angle .theta..sub.MAX. However, this would result in a simultaneous decrease of .theta..sub.MIN at the central portion of the monitor region extension member 2, which would lead to a narrower monitor region at the central portion requiring widest monitor region. This is also a technical problem to be overcome.
Another technical problem lies in that the conventional monitor region extension member 2 has a function to extend the monitor region surrounding the motor vehicle as described above but has by no means a function to compensate the distortion which may occur in the lens 4, so that such an optical distortion is output intactly to the CCD camera.
A further technical problem may also arise that there occurs a difference in a sense of distance between the actual monitor region and a picture of the monitor region derived in the form of output of the CCD camera, with the result that it becomes difficult to view and recognize the position of the obstacle.